It's not just game developers that are buying these types of ad units: E-commerce companies and brands are buying them up because they result in significant downloads and an easily measurable return on investment (the holy grail of marketing). A mobile app install ad doesn't just drive that one-time download, it drives what is known as "lifetime customer value," whereby brands can analyze everything they do in that app.

A huge proportion of Facebook's mobile advertising revenue is made up from mobile app install ads (although Facebook has never publicly declared the percentage.) The Information pegged Facebook's mobile app install ad revenue at $3 billion in 2014, or one-third of its total revenue. The report also claims that Facebook accounts for half of the entire mobile app install ad market.